This invention relates generally to the field of integrated circuits for automatically regulating the gain of a differential amplifier.
In many applications it is desirable to amplify a signal of variable amplitude in a manner such that the level of the amplified signal is substantially constant. For this purpose, amplifiers with variable gains which are regulated according to a feedback mechanism are often used.
In U.S. Pat. No. 5,805,022 to Bruccoleri et al., a circuit is described in which the output of a variable gain differential amplifier drives a full-wave rectifier. The full wave rectifier output signals are used to drive a comparator. This comparator in turn drives a charge pump circuit whose output is converted from voltage-to-current to serve as a regulating signal for controlling the gain of the variable gain differential amplifier (VGA). When this circuit operates on an amplified input signal that is asymmetrical, Bruccoleri et al indicate that it has a disadvantage. Bruccoleri et al indicate that the portion of the asymmetrical signal emerging from the full wave rectifier with the greatest amplitude exclusively controls the gain of the VGA, which can be problematic in some instances.
In order to remedy this perceived shortcoming, Bruccoleri et al provide a gain-regulating circuit which substitutes two half-wave rectifiers for the full wave rectifier. Moreover, each half-wave rectified signal is supplied to its own comparator and charge-pump circuit. The result is a circuit that independently integrates the contribution of the output of each half-wave rectifier to establish an appropriate level of gain for the VGA.
Unfortunately, although this circuit operates well with both symmetrical and asymmetrical signals, the duplication of current sources, comparators and charge pumps required to implement this arrangement uses a large amount of silicon on a semiconductor device. This results in increased cost, size and power consumption and decreased reliability.
While the circuit arrangement of Bruccoleri et al. provides good performance, there is need for other circuit variations that can be used to reduce the amount of silicon consumed on a semiconductor device, for use when amplifying both symmetrical and non-symmetrical signals.
In accordance with one embodiment of the present invention, a circuit which regulates gain in a differential amplifier having first and second outputs, includes a fully differential amplifier receiving the first and second outputs, the fully differential amplifier producing first and second amplified outputs. A comparison circuit compares the first and second amplified outputs with a reference and produces an output signal if either the first or second amplified output exceeds the reference. A processing circuit is connected to receive the output signal from the comparison circuit, and to generate a regulating signal dependent upon a duration of the output signal, wherein the regulating signal regulates a gain of the differential amplifier.
In another embodiment of the present invention, a circuit which regulates gain in a differential amplifier having first and second outputs includes a first switch coupled to the first output, the first switch being controlled by the second output. A second switch is coupled to the second output, the second switch being controlled by the first output. A scaling circuit receives the first and second outputs under control of the first and second switches to produce a scaled output. A comparison circuit compares the scaled output with a reference and produces an output signal if the scaled output exceeds the reference. A processing circuit, connected to receive the output signal from the comparison circuit, generates a regulating signal dependent upon a duration of the output signal wherein the regulating signal regulates the gain of the differential amplifier.
In yet another embodiment of the present invention, a circuit which regulates gain in a differential-amplifier having first and second outputs includes a fully differential amplifier receiving the first and second outputs, the fully differential amplifier having a first gain applied to the first output and a second gain applied to the second output to produce first and second amplified outputs. A comparison circuit compares the amplified first and second amplified outputs with a reference and produces an output signal if either the first or second amplified output exceeds the reference. The comparison circuit includes: a comparator having multiple inputs each of which are compared with a reference, and a processing circuit, connected to receive the output signal from the comparison circuit. The processing circuit generates a regulating signal dependent upon a duration of the output signal. The processing circuit includes: a charge pump receiving the output signal from the comparison circuit and producing a control voltage, and a voltage to current converter receiving the control voltage from the charge pump and producing a control current therefrom. The control current operates as a regulating signal that regulates the gain of the differential amplifier.
A method of regulating a gain of a variable gain differential amplifier having output signals, according to an embodiment of the invention includes: amplifying the output signals to produce amplified signals; comparing the amplified signals with a reference in a comparison circuit; and if either amplified signal exceeds the threshold, generating a regulating signal to reduce the gain of the variable gain amplifier.
Another method of detecting a level of a differential output signal in accordance with one embodiment of the present invention includes: comparing each differential output signal with a reference signal using a single comparison circuit; and if either output signal exceeds the reference signal, generating an output signal.
Many variations, equivalents and permutations of these illustrative exemplary embodiments of the invention will occur to those skilled in the art upon consideration of the description that follows. The particular exemplary embodiments described above should not be considered to limit the scope of the invention.